"val AS name" to "name := val", as per recent discussion.

This patch catches everything in the original named-parameters patch,
but I'm not certain that no other dependencies snuck in later (grepping
the source tree for all uses of AS soon proved unworkable).

In passing I note that we've dropped the ball at least once on keeping
ecpg's lexer (as opposed to parser) in sync with the backend.  It would
be a good idea to go through all of pgc.l and see if it's in sync now.
I didn't attempt that at the moment.